This is a trick I used to use 3/4 years ago when we were just getting into Walmart and we had to add items to the catalog.
Can’t guarantee it will work 100% of the time but I just tried it and it worked on all listings.
Go on the page of the listing you want to add to your catalog.
Right-click any whitespace on the listing and choose “View Page Source,” a common keyboard shortcut is ‘CTRL + U’
It will open the HTML source if it didn’t open automatically.
‘CTRL + F’ and search for ‘UPC’ and/or ‘GTIN’ (without quotes)
The UPC/GTIN should be right next to the ID of the found results.
I’m new here so I don’t know if I am allowed to post links or screenshots, but the results should look something like:
“upc”:“030772097502”
or
“sku”:“7232106224”,“gtin13”:“030772097502”